NWS STORM PREDICTION CENTER NORMAN OK
0100 AM CDT SUN AUG 31 2014
VALID 311200Z - 011200Z
...THERE IS A SLGT RISK OF SVR TSTMS FROM THE CENTRAL PLAINS TO THE
UPPER MS VALLEY...
...THERE IS A SLGT RISK OF SVR TSTMS FROM PART OF THE MID-ATLANTIC
STATES TO SRN NEW ENGLAND...
CORRECTED TO INCLUDE THE EASTERN SLIGHT RISK AREA IN THE SUMMARY.
...SUMMARY...
SEVERE THUNDERSTORMS ARE EXPECTED TO DEVELOP ALONG A COLD FRONT
SUNDAY AFTERNOON AND EVENING FROM MINNESOTA SOUTHWESTWARD ACROSS
WESTERN IOWA...AND CENTRAL AND EASTERN NEBRASKA INTO KANSAS. LARGE
HAIL...DAMAGING WINDS...AND A FEW TORNADOES ARE EXPECTED WITH THIS
ACTIVITY. A FEW SEVERE STORMS WILL ALSO BE POSSIBLE THIS AFTERNOON
AND EARLY EVENING ACROSS NORTHERN VIRGINIA AND MARYLAND TO SOUTHERN
NEW ENGLAND.
...SYNOPSIS...
A BROAD NWRN STATES UPPER TROUGH WILL AMPLIFY TODAY ACROSS THE NRN
AND CENTRAL PLAINS. A PROGRESSIVE SHORTWAVE TROUGH...CURRENTLY
MOVING EWD THROUGH THE NRN INTERMOUNTAIN WEST PER WATER VAPOR
IMAGERY...WILL TRANSLATE THROUGH THE PARENT LARGE-SCALE
TROUGH...REACHING THE CENTRAL DAKOTAS/NEB BY 01/00Z. THIS MIDLEVEL
FEATURE WILL THEN MOVE THROUGH THE MID MO VALLEY...REACHING THE
UPPER MS VALLEY BY 12Z MONDAY. A COLD FRONT WILL ADVANCE EWD ACROSS
THE UPPER MIDWEST...WHILE THE TRAILING PORTION MOVES SEWD ACROSS
NEB/KS.
AN EXTENSIVE CORRIDOR OF RICH TROPICAL MOISTURE /PW EXCEEDING 2
INCHES/ WILL SPREAD THROUGH THE OH VALLEY AND MID ATLANTIC STATES TO
SRN NEW ENGLAND TODAY. A SHORTWAVE TROUGH...EMBEDDED WITHIN THIS
MOISTURE PLUME...WILL TRACK FROM KY/TN TO CENTRAL PA/SRN NY BY LATE
THIS AFTERNOON/EARLY EVENING...AND THEN UNDERGO SUBSTANTIAL
WEAKENING AS IT MOVES INTO SRN NEW ENGLAND LATE SUNDAY NIGHT. WEAK
HEIGHT FALLS WILL PRECEDE THE KY/TN SHORTWAVE TROUGH ACROSS THE
CENTRAL APPALACHIANS TO SRN NEW ENGLAND...AS A COMPACT SHORTWAVE
TROUGH MOVES EWD FROM LOWER MI TOWARD NRN NEW ENGLAND THIS FORECAST
PERIOD.
...UPPER MS VALLEY TO CENTRAL PLAINS...
EARLY IN THE PERIOD...ELEVATED THUNDERSTORMS WILL LIKELY BE ONGOING
ALONG THE NOSE OF LLJ ACROSS PORTIONS ERN ND AND NWRN MN. THIS
ACTIVITY SHOULD BE ROOTED AT OR ABOVE 850MB WITH THE GREATEST THREAT
WITH THESE STORMS BEING MARGINALLY SEVERE HAIL BEFORE THEY PROPAGATE
INTO SRN MANITOBA AND ADJACENT NRN ONTARIO ATTENDANT TO A
PROGRESSIVE LEAD SHORTWAVE TROUGH AND SURFACE LOW.
A PLUME OF STEEP MIDLEVEL LAPSE RATES WILL SPREAD E/NEWD FROM THE
CENTRAL PLAINS TO THE UPPER MS VALLEY. MEANWHILE...SOUTHERLY
LOW-LEVEL WINDS AHEAD OF THE COLD FRONT WILL ADVECT MOISTURE NWD
WITH SURFACE DEW POINTS IN THE MID-UPPER 60S SPREADING INTO ERN
NEB/IA TO THE UPPER MS VALLEY. THESE FACTORS COMBINED WITH SURFACE
HEATING WILL RESULT IN STRONG INSTABILITY /MLCAPE 2000-3000 J PER
KG/ ACROSS THE CENTRAL PLAINS TO CENTRAL MN.
TSTMS MAY DEVELOP BY EARLY-MID AFTERNOON ALONG CENTRAL NEB PORTION
OF THE COLD FRONT. THIS WILL OCCUR AS FORCING FOR ASCENT ATTENDANT
TO WEAK HEIGHT FALLS WITH THE LARGE-SCALE TROUGH AND AHEAD OF THE
PROGRESSIVE NRN INTERMOUNTAIN WEST SHORTWAVE TROUGH REACH THIS AREA.
EFFECTIVE BULK SHEAR WILL INCREASE ACROSS NEB/NRN KS AS A 50-60 KT
WLY MIDLEVEL JET EMERGES FROM THE CENTRAL ROCKIES. STRONGER SURFACE
HEATING FROM CENTRAL NEB INTO KS RESULTING IN A DEEPLY MIXED
BOUNDARY LAYER WILL TEND TO RESULT IN DAMAGING WINDS AND LARGE HAIL
BEING THE PRIMARY THREATS IN THE CENTRAL PLAINS. GIVEN VERY STEEP
LAPSE RATES /EXCEEDING 8 C/KM IN THE 700-500-MB LAYER/...STRONG
INSTABILITY AND EFFECTIVE BULK SHEAR OF 40-50 KT SUGGESTS VERY LARGE
HAIL WILL BE POSSIBLE.
MEANWHILE...STORMS ARE EXPECTED TO DEVELOP ALONG THE REST OF THE
COLD FRONT BETWEEN 21-00Z FROM MN THROUGH ERN SD...ERN NEB...AND WRN
IA WITH BOTH QUASI-LINEAR AND SUPERCELL STORM STRUCTURES. ALL
SEVERE HAZARDS CAN BE EXPECTED. THE GREATEST SEVERE WEATHER THREAT
IS EXPECTED ACROSS ERN NEB INTO WRN/CENTRAL IA AND SRN MN FROM LATE
THIS AFTERNOON INTO THE EARLY EVENING. THIS REGION WILL HAVE A
JUXTAPOSITION OF A STRENGTHENING SSWLY LLJ ENHANCING LOW-LEVEL
SHEAR...LOWER LCLS...AND EFFECTIVE BULK SHEAR VECTORS CROSSING THE
COLD FRONT SUPPORTING SOME DISCRETE SUPERCELLS AND/OR EMBEDDED
SUPERCELLS WITHIN THE DEVELOPING LINEAR STRUCTURE. STORM MERGERS
INTO THE EVENING SHOULD RESULT IN AN ELONGATED MCS-TYPE COMPLEX
ADVANCING EWD ACROSS THE UPPER MS TO LOWER MO VALLEYS.
...NRN VA/MD/ERN PA/NRN NJ/SERN NY/SRN NEW ENGLAND...
A CORRIDOR OF MODERATE INSTABILITY /MLCAPE UP TO 1000-1500 J PER KG/
IS EXPECTED TO DEVELOP THIS AFTERNOON FROM NRN VA/MD TO SRN NEW
ENGLAND...DESPITE THE PRESENCE OF WEAK MIDLEVEL LAPSE RATES. THIS
DESTABILIZATION WILL BE LOCATED ALONG THE ERN PERIPHERY OF EARLY DAY
CLOUDS/PRECIPITATION EXPECTED TO EXTEND FROM THE OH VALLEY INTO
CENTRAL PA/NY. CONVERGENCE IN VICINITY OF A SW-NE ORIENTED
DIFFERENTIAL HEATING BOUNDARY AND WEAK HEIGHT FALLS SPREADING ACROSS
THIS REGION SHOULD PROVE SUFFICIENT FOR DEVELOPING TSTMS FROM
AFTERNOON INTO THE EARLY EVENING. EFFECTIVE BULK SHEAR OF 30-40 KT
WILL SUPPORT ORGANIZED STORMS WITH DAMAGING WINDS BEING THE PRIMARY
THREAT AS ACTIVITY SPREADS FROM WEST TO EAST.

A level 2 was issued for northern Italy, Slovenia and Croatia mainly for excessive convective rainfall, large hail, and isolated tornado chances.

A level 1 was issued for Austria, Czechia and southwestern Poland mainly for large hail.

A level 1 was issued for Turkey mainly for large hail and severe wind gusts.

A level 1 was issued for northern Netherlands and Germany, Denmark and adjacent sea mainly for spout-type tornadoes.

A level 1 was issued for northern parts of Morocco, Algeria and Tunesia mainly for severe wind gusts and large hail.

A level 1 was issued for eastern Belarus mainly for large hail.

SYNOPSIS

An upper level low will move from the North Sea across Germany towards the northern Mediterranean where lee cyclogenesis will occur across N Italy. A strong surface cold front will make its way across Germany towards the Adriatic sea, where it will trigger thunderstorms with a major threat of excessive precipitation during evening and night. A weaker mid level cold front passes Spain from north to south.Another upper low with associated slight instability resides over Belarus. Another mid level trough slowly passes over western Turkey.

NWS STORM PREDICTION CENTER NORMAN OK
0750 AM CDT SAT AUG 30 2014
VALID 301300Z - 311200Z
...THERE IS A SLGT RISK OF SVR TSTMS LATE THIS AFTERNOON/EVENING
ACROSS THE NRN HIGH PLAINS...
...SUMMARY...
A FEW SEVERE STORMS ARE POSSIBLE LATE THIS AFTERNOON AND EVENING
ACROSS PARTS OF THE NORTHERN HIGH PLAINS. OTHER STRONG STORMS ARE
POSSIBLE TODAY IN A CORRIDOR ARCING FROM THE CENTRAL GULF COAST INTO
THE TENNESSEE VALLEY...AND ACROSS LOWER MICHIGAN AND ADJACENT
PORTIONS OF THE GREAT LAKES REGION.
...NRN ROCKIES TO NRN HIGH PLAINS THIS AFTERNOON INTO TONIGHT...
A MIDLEVEL TROUGH AND ASSOCIATED BELT OF 35-50 KT FLOW WILL PROGRESS
EWD OVER THE NRN GREAT BASIN/ROCKIES TO THE NRN HIGH PLAINS BY
TONIGHT. AN ACCOMPANYING COLD FRONT WILL MOVE ACROSS WY/MT THIS
AFTERNOON/EVENING...AS LEE CYCLOGENESIS IN NE WY DRAWS A NARROW
CORRIDOR OF RESIDUAL BOUNDARY LAYER MOISTURE NWWD INTO THE NRN HIGH
PLAINS. INITIAL CONVECTION WILL FORM WITHIN THE BAND OF ASCENT
PRECEDING THE MIDLEVEL WAVE...AND STORMS SHOULD STRENGTHEN LATE THIS
AFTERNOON/EVENING WHILE ENCOUNTERING THE RICHER MOISTURE ON THE HIGH
PLAINS E/NE OF THE BIG HORNS.
SURFACE HEATING/MIXING NEAR THE LEE CYCLONE AND ALONG THE COLD FRONT
WILL REDUCE CONVECTIVE INHIBITION...WHILE MIDLEVEL LAPSE RATES
GREATER THAN 7 C/KM AND BOUNDARY LAYER DEWPOINTS IN THE 50S RESULT
IN MLCAPE UP TO 1500 J/KG. IN COMBINATION WITH INCREASING VERTICAL
SHEAR...THE THERMODYNAMIC PROFILES WILL SUPPORT ORGANIZED
CLUSTERS/SUPERCELLS WITH AN ATTENDANT RISK FOR DAMAGING WINDS AND
LARGE HAIL BEFORE CONVECTION WEAKENS TONIGHT IN THE CENTRAL DAKOTAS
WHERE BUOYANCY WILL BE WEAKER.
...LOWER MI/NRN INDIANA THIS AFTERNOON...
A MIDLEVEL TROUGH OVER NW IL/SW WI...AND AN ASSOCIATED SURFACE LOW
IN CENTRAL WI...WILL MOVE ENEWD TO NRN LOWER MI BY THIS EVENING.
LOW-LEVEL CONVERGENCE/ASCENT WILL BE FOCUSED NEAR THE LOW AND A
TRAILING WEAK COLD FRONT AS DAYTIME HEATING REMOVES CONVECTIVE
INHIBITION AND 68-70 F DEWPOINTS SPREAD NEWD INTO LOWER MI.
SCATTERED THUNDERSTORM DEVELOPMENT IS EXPECTED ALONG THE WIND SHIFT
THIS AFTERNOON...WHEN ORGANIZED CLUSTERS/MARGINAL SUPERCELLS COULD
PRODUCE ISOLATED DAMAGING GUSTS GIVEN MODERATE BUOYANCY...EFFECTIVE
BULK SHEAR NEAR 35 KT...AND A BELT OF 40 KT FLOW IN THE 850-500 MB
LAYER FOR POTENTIAL DOWNWARD MOMENTUM TRANSFER IN HEAVY
PRECIPITATION CORES. POOR MIDLEVEL LAPSE RATES WILL TEND TO LIMIT
UPDRAFT STRENGTH AND ANY HAIL RISK...THOUGH LOWER MI WILL BE
MONITORED CLOSELY FOR THE POSSIBILITY OF A LOW-END SLIGHT RISK IN
LATER OUTLOOK UPDATES.
...LA AREA TODAY...
A PLUME OF TROPICAL MOISTURE /PW VALUES AOA 2.25 INCHES AND LOW-MID
70S DEWPOINTS/ HAS SPREAD INLAND FROM THE NW GULF TO LA IN
ASSOCIATION WITH A WEAK MIDLEVEL TROUGH NEAR THE SABINE RIVER. THE
MOIST ENVIRONMENT IS SUPPORTING SCATTERED THUNDERSTORMS THIS
MORNING...WHILE A BELT OF 30-35 KT SLY FLOW NEAR 850 MB IS
CONTRIBUTING TO SOME ENHANCEMENT OF LOW-LEVEL SHEAR /EFFECTIVE SRH
OF 150-250 M2 PER S2/. THIS ENVIRONMENT IS SUPPORTIVE OF SOME
WEAKLY ROTATING STORMS AND THE RISK FOR A BRIEF TORNADO AND/OR
DAMAGING GUST FROM THIS MORNING INTO EARLY AFTERNOON AS THE SYSTEM
MOVES NNEWD.
..THOMPSON/MOSIER.. 08/30/2014

A level 1 was issued for S-Norway and the Kattegat mainly for excessive rain and an isolated tornado event.

A level 1 was issued for E-CNTRL Spain mainly for an isolated large hail event.

A level 1 was issued for NW Italy mainly for isolated large hail.

SYNOPSIS

Ex-hurricane's (CRISTOBAL) WAA induces rapid mid-layer thickness increase west of Ireland (e.g. 500 hPa height increase in excess of 10 gpdm/12h). This causes a sharpening thermal gradient over Ireland and UK between the ridge and a stationary vortex over N-Scotland. Both, the northward building ridge and intensifying mid/upper jet winds along the SW fringe of that vortex cause the vortex's slow SE-ward motion during the overnight hours ... approaching Benelux from the NW during the end of the forecast. Advection of moist marine air ahead of that low result in marginal thunderstorm conditions for this forecast period.

Positive tilted trough over NE Europe features cold mid-levels, but no distinct moisture source is available to feed any storms. Hence scarce thunderstorm probabilities exist.

A level 1 was issued for Northeastern Spain mainly for large hail and to a lesser extent for severe wind gusts.

A level 1 was issued for Southern France mainly for excessive precipitation.

A level 1 was issued for the coastline of Scotland mainly for tornados.

SYNOPSIS

Two long wave troughs influence the forecast area. One of them is situated over far Eastern Europe / Western Russia bringing rather cool airmasses and unstable conditions to this region. The second trough is influencing Western Europe. Its center is situated over Scotland and moves slowly eastward. A surface low having a perpendicular vertical axis corresponds to it. Its frontal system is influencing Eastern Germany and Poland.

Both troughs are separated by a weak ridge. Its axis is moving from Eastern Germany into Eastern Europe during the forecast time. It is mostly overrun by warm air advection in higher levels and has only weak impact since a frontal system can be found at almost the same place, as described above.

A cut of high that has a loose connection to the ridge, leads to sinking motion and stable conditions over parts of Northern Europe.

Southern Europe is lying south of the frontal zone and is influenced by warm and humid airmasses. A strong capping can be found over most of the Mediterranean. It is somewhat weaker over continental areas.

A level 2 was issued for E France and S Germany mainly for tornadoes and severe wind gusts.

A level 1 was issued for NE/E France, Belgium, Luxembourg, S-central Germany and W Czech mainly for severe wind gusts and to a lesser extent for tornadoes.

A level 1 was issued for S UK for a chance of tornadoes.

A level 1 was issued for parts of the North Sea and Baltic Sea for spout-type tornadoes.

SYNOPSIS / DISCUSSION

... SE UK, NE / E France, SW / central Germany ...

A dissipating low pressure system over the British Isles which continues eastwards advects warm and unstable air into W / central Europe. A few hundred J/kg of SBCAPE are forecast consistently in the warm sector by the past runs of GFS and ECMWF, at least partly overlapping with 15 - 25 m/s of 0-6 km bulk shear. T/Td is forecast around 20/16 °C for SW Germany which leads to low LCL heights. Convection should fire up in the late morning / early afternoon hours even though there is some subsidence over the central parts of Germany from the upper ridge. Combined with enhanced LL shear and SRH1, this may allow a few tornadoes. Further north, LL shear is weaker but the background wind field should be intense enough to support isolated severe wind gusts mainly from downward transfer of momentum. Local heavy precipitation is possible but fast storm motion should preclude a risk of flooding in most cases.

Over UK, there is very weak LL shear forecast in the afternoon hours but a weak background flow should support some spout-type tornadoes.

Sufficient buoyancy and weak background winds enhance the chance for long-lasting convergent flow in the vicinity of convective storms and may lead to funnels which could end up in spout-type tornadoes. The greatest chance for landspouts / waterspouts exists around Denmark and N Poland. Further north, deep layer shear is also quite weak but the stronger background flow will likely disturb the formation of funnels / tornadoes.

Some regions with locally enhanced LL shear exist over Finland but overall instability is likely too low to support a tornado threat covered by a LVL1.

... NW Italy and Mediterranean Sea...

A plume of unstable air is advected from the Balearic Islands towards Italy. It will likely remain strongly capped but in case of convective initiation, isolated storms may profit from 20 - 25 m/s of 0-6 km bulk shear and could develop mesocyclones. Overall threat is likely too marginal for a threat level as the capping inversion is likely too strong for storm development.

A level 1 was issued for some coastlines of Denmark, S Sweden and Poland for waterspouts.

SYNOPSIS and DISCUSSION

Two pronounced cyclones are placed over the British Isles and from Southern Scandinavia into the Baltic region. A strong zonal flow is present to their South, but temporary high pressure and dry, mostly cool air create often hostile conditions for deep convection.

Convection in the Northern half of Europe will mostly take place in well-mixed, weakly sheared polar air in the periphery of the two cyclones. Late in the forecast period, limited area models show well-marked convergence zones between Sweden and Denmark and along the Polish coast which could favor the development of a few waterspouts.

In the greater Mediterranean region, regionally rich moisture is still present but very weak lapse rates allow only limited patches of CAPE. Isolated to scattered showers are expected in the morning over the Adriatic Sea, the Northern Aegean Sea and the Bosporus (including the possibility of one or two waterspouts), and in the afternoon over orographic features of the Southern Balkans. However, forecast instability seems too shallow to produce a lot of lightning or to benefit from 15 to 20 m/s deep-layer shear.

An overlap of more robust CAPE and shear would be present over the Black Sea, but a strong cap will likely suppress convection.